Methods are currently available for providing non-removable and optically readable security devices in paper during the paper making process. U.S. Pat. No. 4,437,935 discloses a method for incorporating one such security device by using a carrier paper which becomes dispersed upon contact with the wet paper stock during the paper making process. U.S. patent application (Crane-4) discloses a method of introducing a security device within the paper stock during the paper making process by employing a carrier paper to which the device is adhered. The carrier paper intermeshes with the base paper fibers forming a composite paper facilitating permanent attachment of the device. Both the aforementioned Application and Patent are incorporated herein for purposes of reference.
In both the aforementioned patent and patent application the carrier paper used to transport the security device remains an integral part of the finished paper and can be discerned from the base paper only by close examination. When it is desired to provide a micro-code integrally formed within the substance of the paper for optical reading by means of transmitted light, the carrier paper is an inappropriate substrate for fine line codes or microprint. U.S. Pat. No. 3,880,706 discribes a method for imparting security fibers manufactured from a thermoplastic material which is fused within the paper fibers during the paper making process. One such material being a thermoplastic material which becomes fused at the final stages of the paper making process by subjecting the paper to a pre-determined temperature. Once the thermoplastic material has become fused within the paper, its presence may be detected by transmitted light.
When micro-coded information is to be deployed within paper, it is first microprinted on a thin strip or transparent material such as a polyester plastic film. The polyester film is then introduced from a continuous spool to the paper stock on a Fourdrenier or a cylinder mold machine during the sheet forming process. Once the paper fibers are pressed and dried to form the finished paper, the polyester film remains intact and the microprinted material can be viewed by transmitted light. Such a film, is removable from the paper by tearing the paper to expose the film and then lifting the film from the paper as a continuous strip. The authenticity of a banknote or security document can be brought into question if part or all of the encoded thin film strip has been removed. If the micro-code contains machine-readable information for both verifying the authenticity of currency, for example, as well as identifying the denomination of the currency, this could present a serious problem. The identifying micro-code for a higher denomination currency could be reinserted.
The purpose of this invention is to provide a method for incorporating micro-coded information within security paper without leaving any indication of a carrier paper or of a carrier film.